Two suppressors, sup-7(st5)X and sup-5(el464)III of the small nematode Caenorhabditis elegans, have recently been proven to result in an altered tRNA that promotes the readthrough of UAG amber terminators (Wills et al., in press). Indirect evidence indicates that sup-7(st5) is a tRNATrp with its anticodon altered to read UAG terminators. The proposed research will define the sup-7(st5) mutation through recombinant DNA and DNA sequencing studies. Other suppressors including sup-5(el464) will also be analyzed at the molecular level. Using classical genetic approaches developed in the study of nonsense suppressors in bacteria and yeast, we will recover additional nonsense suppressors for the UAG terminator and for UAA and UGA terminators. Methods will be developed to apply nonsense suppressors in the study of other problems in C. elegans biology. These include the isolation of conditional mutations, the determination of the site of gene expression and the development of DNA transfer techniques. Nonsense suppressors have great potential as genetic tools in the study of C. elegans. The knowledge of suppressors gained in this simple animal should be useful in the application of nonsense suppressors to the study and treatment of mutations in other animals including man.